The present invention is related to flat panel electro-optic display systems, and more particularly to an improved panel electrical circuit structure and drive system for exciting individual display elements to the desired brightness.
A thin film transistor controlled flat panel electro-optic display panel is described in IEEE Transactions On Electron Devices, September, 1975. The display medium described therein is an electroluminescent (EL) phosphor. The display panel comprises an array of individually controllable display elements. A thin film transistor control circuit is associated with each display element, with the display element and circuitry interconnected by X--Y buses, which permit peripheral addressing. In such earlier display panels, at least two buses were used in either the horizontal or vertical direction to permit a reference level connection or return path for the individual EL display element. The brightness of the individual EL display element was controlled by varying the amplitude of the information signal potential which was applied to the gate of a drive signal switching transistor. The conductance of this transistor and the resultant current amplitude through the EL display medium was controlled as a direct function of the information signal amplitude. This drive and control system places a stringent requirement on the thin film transistor threshold uniformity and stability across the display panel for accurate grey scale display. A typical panel would have thousands of display elements and transistors. The drive system also requires a higher than desired power dissipation at intermediate brightness. This is because the current flow in the drive control transistor is in phase with the excitation voltage across it. Low power dissipation is a key to portability and versatility of the lightweight display panel since it reduces the size and weight of the power pack required.
The present invention involves a change in the display panel circuitry and in the drive system. Power dissipation is eliminated in the ideal case by transferring brightness control from the current amplitude to the duty cycle or on-time domain. The threshold sensitivity requirement is reduced by lowering the effective gain of the control transistor.